Rapid removal of plutonium from the oceanic surface layer by zooplankton faecal pellets

Abstract

IT is possible that increasing quantities of plutonium will be introduced into the marine environment, so detailed knowledge of the oceanic behaviour of this element is highly desirable. The levels of plutonium in sea water and marine organisms have been established in a general way^1,2, and suggestions have been made as to possible transport mechanisms. Noshkin and Bowen³ proposed a model which associates plutonium with a mixed population of particles, sinking at rates of 70–392 m yr⁻¹, and Folsom et al.⁴ suggested that algae and, perhaps, the main phytoplanktonic biomass may have an important role in determining the chemical and physical forms of plutonium predominant in the ocean. The role of the zooplanktonic biomass has not been investigated in detail, but it is known that for several elements Zooplankton metabolism may be an important biological factor in the removal of the elements from the surface layer of the ocean to depth^5–9. The particular importance of Zooplankton faecal pellets in this process has been highlighted, and the faecal pellets of a common zooplanktonic species, the euphausiid Meganyctiphanes norvegica, have been shown to be rich in the important naturally-occurring alpha-emitter ²¹⁰Po when compared with whole organism levels¹⁰. We have performed a similar study for plutonium, and we report here that M. norvegica faecal pellets are indeed relatively rich in plutonium, and suggest that Zooplankton faecal pellet deposition might be an important vector in the vertical oceanic transport of the element.